12,000' probably has an internal cabin pressure of around 3,000' to 3,500' and of course it's unlikely to be at exactly 12,000' during detonation.
Turbulence can cause massive stress on an airframe so their tougher than you would think. https://www.youtube.com/watch?v=rak2HldVp9M
It's like saying a rope that's living 500 pounds is under 5x the load as one lifting 100 pounds while ignoring it's designed to lift cars. And for a rather extreme example: http://www.aloha.net/~icarus/index.htm 30' just gone.
There have been some spectacularly involved structural failures, with Aloha Airlines 243 perhaps the most impressive of these.
https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
Crucially important:
1. No major structural members were involved. The aircrafts main beams run beneath the cabin and cargo floors. As it was, loss of structural integrity of the outer envelope introduced significant additional flex to the airframe which the pilots had to compensate for.
2. No critical control systems were involved. In the Pan Am 103 case, steering cables were severed.
3. There was no internal overpressure or extended hull damage. In AA243, the initial rupture was the result of corrosive metal fatigue and delamination of the outer skin.
The point is without that knowledge and access by the time you have enough HE to take down a modern jumbo jet you can also use it to kill a very large number of people in other ways.
Remember, most bombings are really poorly executed at a technical level. Army's use grenades because they work. If you want to do as much damage to a crowd as possible your better off with a large number of fragmentation grenades than a suicide vest. Create a choke point, detonate something to create a crush, blow up the densely packed crowd.
PS: Mortars are frightfully effective yet tiny.
"Passengers on a Daallo Airlines flight leaving from Mogadishu were subjected to just about the most terrifying experience imaginable on Tuesday as a bomb exploded in the plane's passenger cabin somewhere above 12,000 feet."
Note that while 12,000 feet is well above sea level, it's only 4,000 feet above effective internal cabin altitude of 8,000'. So the fuselage stresses are vastly lower than those at 29,000'. The pressure differential is about 10 pa (pascals), vs. about 40 pa at 29,000'. That's roughly 1.5 psi vs. 11.5 psi -- there's nearly 10x the pressure differential at FL29.